A variety of methodologies have been employed in an attempt to impregnate polymers, and in particular thermoplastic polymers, with various impregnation additives. For example, certain polymers can be impregnated with selected additives by immersing the polymers in a solution comprised of the additives for an extended period of time. In addition, it may also be possible to incorporate the additives into polymers during melt processing and/or extrusion. Furthermore, additives may be impregnated into polymers by dissolving the additives into various compounds, such as CO.sub.2, N.sub.2 O, and ethylene, maintained at or near their supercritical temperatures and pressures, and contacting this mixture with the polymer or polymers to be impregnated. Above a defined temperature and pressure, these pressurized compounds form supercritical fluids that serve both as swelling agents for the polymers to be impregnated, and as volatile solvents for additives to be impregnated into the polymers.
Existing methods of impregnating polymers with additives using supercritical fluids are limited by the requirement that the selected additive or additives be soluble in the supercritical fluid, and that the mixture of the additive solubilized in the supercritical fluid be compatible with (i.e. soluble in) the polymer to be impregnated. For example, U.S. Pat. No. 4,598,006 (Sand) discloses a method for impregnating a thermoplastic polymer with an impregnation material (i.e. a fragrance, a pest control agent, or a pharmaceutical composition) by dissolving the impregnation material in a volatile swelling agent (e.g., CO.sub.2 maintained at or near supercritical conditions, swelling the thermoplastic polymer by contacting it with the supercritical or nearly supercritical volatile swelling agent containing the impregnation material, and reducing the pressure so the volatile swelling agent diffuses out of the thermoplastic polymer. Among other limitations, Sand teaches that the impregnation material must be soluble in the volatile swelling agent, and that the volatile swelling agent be compatible with (i.e. soluble in) the polymer to be impregnated. Given the lipophilic nature to the volatile swelling agents and polymers disclosed in Sand, the impregnation materials disclosed in Sand are also lipophilic. See also, U.S. Pat. No. 4,678,684; EPO Patent Application Nos. 0 200 197, 0 401 713, 0 405 284; and Australian Patent Application No. 57091/86.
Similarly, U.S. Pat. No. 4,820,752 (Berens et al.) discloses a process for infusing an additive into a polymer by dissolving the additive into a compressed normally gaseous fluid solvent (e.g., CO.sub.2) that has a boiling point below room temperature and a density of at least 0.01 g/cc, contacting the solution of the additive and normally gaseous fluid solvent with a polymeric material for a time sufficient to allow at least part of the solution to be absorbed into the polymeric material, and separating the normally gaseous fluid solvent from the polymeric material leaving the additive infused within the polymeric material. Importantly, Berens et al. discloses that the additive must have some degree of solubility in the compressed fluid, and the solution of the compressed fluid and additive must have some degree of solubility in the polymeric material. See also, EPO Patent Application No. 0 222 207.
In addition, supercritical fluids have also been used as a solvent to re-impregnate aromatic components into a tea residue after the caffeine component of the tea had been extracted (U.S. Pat. No. 4,167,589; Vitzthum et al.), as a solvent during the preparation of substance embedded microspheres, by dissolving a substance and polymeric carrier, with or without a liquid medium, into a supercritical gas (U.S. Pat. No. 5,043,280; Fisher et al.), and as a solvent for various monomers or polymers to be impregnated into porous materials, such as wood, to increase the strength and other properties of the porous materials (U.S. Pat. Nos. 4,992,308 and 5,169,687; Sunol).
None of the previously disclosed methods can be used to successfully impregnate additives, and in particular hydrophilic additives, into polymers when the additives are incompatible with (i.e. substantially insoluble in) the supercritical fluid. In fact, to date, no method has been provided for the impregnation of additives into polymers when such additives are substantially insoluble in the supercritical fluid.